If None, then compute at 200 frequencies around the interesting parts
of the response curve (determined by pole-zero locations). If a single
integer, then compute at that many frequencies. Otherwise, compute the
response at the angular frequencies (e.g. rad/s) given in worN.

plot:callable, optional

A callable that takes two arguments. If given, the return parameters
w and h are passed to plot. Useful for plotting the frequency
response inside freqs.